I have a son that's new to camping/scouting, and I'm trying to teach him a thing or two. Knots were never my strong suit, and I'm wondering which one's you think are essential to UL backpacking. Also, if you have a favorite knot book I'd like to hear about it.

Exploding bowline.. very, very useful. Can be used instead of a bowline (works best with thin rope), works the same way, except you can adjust the loop length easily, and it comes undone by pulling on the right line. Not too well known, but it's really easy to tie, I found a video online - http://www.fcff.org/images/Learn%20Images/exploding%20bowline.avi